From the early stage of providing voice-oriented services, a mobile communication system has evolved into a high-speed, high-quality wireless packet data communication system to provide data and multimedia services. Various mobile communication standards, such as High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), Long Term Evolution-Advanced (LTE-A), High Rate Packet Data (HRPD) of the 3rd generation partnership project-2 (3GPP2), and IEEE 802.16, have recently been developed to support high-speed and high-quality wireless packet data transmission services. In particular, the LTE system, which is a system developed to efficiently support high speed wireless packet data transmission, maximizes wireless system capacity by using various wireless access technologies. The LTE-A system, which is an advanced wireless system evolved from the LTE system, has enhanced data transmission capability, as compared to the LTE system.
The existing 3rd generation wireless packet data communication systems, such as HSDPA, HSUPA and HRPD, use technologies of an Adaptive Modulation and Coding (AMC) scheme and a channel-sensitive scheduling scheme to improve the transmission efficiency. With the use of the AMC scheme, a transmitter may adjust an amount of transmission data according to a channel status. That is, when the channel status is not good, the transmitter may reduce the amount of transmission data to adjust the reception error probability to a desired level. In contrast, when the channel status is good, the transmitter may increase the amount of transmission data to achieve efficient transmission of a large quantity of information while adjusting the reception error probability to a desired level.
With the use of the channel-sensitive scheduling-based resource management method, the transmitter selectively provides a service to a user having a good channel status among a plurality of users, and thus, the system capacity increases as compared to the method of assigning a channel to one user and providing a service to the user with the assigned channel. Such a capacity increase as in the above description is referred to as “multi-user diversity gain”. In short, the AMC scheme and the channel-sensitive scheduling scheme are methods that apply an appropriate modulation and coding scheme at a point in time that is determined to be most efficient based on partial channel status information fed back from a receiver.
The AMC scheme, when used together with a Multiple Input Multiple Output (MIMO) transmission scheme, may include a function of determining a rank or the number of spatial layers of a transmission signal. In this instance, to determine an optimal data rate, the AMC scheme may consider the number of layers to which transmission is executed using the MIMO, instead of considering merely a coding rate and a modulation scheme.
In general, the LTE and the LTE-A systems use an Orthogonal Frequency Division Multiple Access (OFDMA) scheme. The OFDMA scheme allocates or manages time-frequency resources through which data or control information is carried for each user, to not overlap one another, that is, to have orthogonality, and thereby distinguishes data or control information of each user. It is known that an increase in a capacity may be expected from the OFDMA scheme as compared to a Code Division Multiple Access (CDMA) scheme that has been used in the existing 2nd and 3rd generation mobile communication systems. One of the several causes bringing about the capacity increase in the OFDMA scheme is that the OFDMA scheme may perform scheduling in a frequency domain (Frequency Domain Scheduling). Although a capacity gain is acquired according to the time-varying channel characteristic using the channel-sensitive scheduling method, it is possible to obtain a higher capacity gain with use of the frequency-varying channel characteristic. According to the conventional art, a mobile communication system formed of a plurality of cells, provides a mobile communication service utilizing the above described various methods.
FIG. 1 illustrates a configuration of a cellular mobile communication system according to the conventional art. Referring to FIG. 1, in a mobile communication system formed of three cells 100, 110, and 120, a transceiving antenna 130 is disposed in the center of each cell 100, 110 and 120. Each cell executes mutual communication with terminals included in a corresponding cell.
The mobile communication system of FIG. 1 includes the first cell 100, the second cell 110, and the third cell 120. The first cell 100 of the cells includes a central antenna 130 disposed in the center of the first cell 100, a first terminal (User Equipment (UE) or Mobile Station (MS)) 140, and a second terminal 150. The central antenna 130 provides a mobile communication service with respect to the two terminals 140 and 150 located in the first cell 100. A distance from the first terminal 140, which is provided with a mobile communication service through the central antenna 130, to the central antenna 130 is relatively farther than a distance from the second terminal 150 to the central antenna 130. Accordingly, a data transmission speed supported to the first terminal 140 is relatively lower than a data transmission speed supported to the second terminal 150.
The mobile communication system as shown in FIG. 1 transmits a Reference Signal (RS) for measuring a downlink channel status of each cell. The RS is also referred to as a pilot signal. In the case of the LTE-A system of the 3GPP, a terminal measures a channel status between a base station and the terminal by using a Channel Status Information Reference Signal (CSI-RS) transmitted by the base station.
FIG. 2 is a diagram illustrating a location of a CSI-RS that a base station transmits to a terminal in an LTE-A system according to the conventional art.
In FIG. 2, a horizontal axis indicates a time domain and a vertical axis indicates a frequency domain. A minimum transmission unit of the time domain is an OFDMA symbol. A single slot 222 and 223 is formed of NsymbDL OFDM symbols. A single subframe 224 is formed of two slots. A minimum transmission unit of the frequency domain is a sub-carrier and an entire system transmission band is formed of a total of NBW sub-carriers. NBW has a value, which is proportional to a system transmission band. A base unit of a resource in time-frequency domains is a Resource Element (RE). The RE is defined by an OFDM symbol index and a sub-carrier index. A Resource Block (RB) 220 and 221 is defined by NsymbDL successive OFDM symbols in the time domain and NscRB successive sub-carriers in the frequency domain. Accordingly, a single RB is formed of NsymbDL×NscRB REs. In general, a minimum transmission unit of data or control information is an RB unit.
A downlink control channel is transmitted within the first 3 OFDM symbols of a subframe. A Physical Downlink Shared Channel (PDSCH), which is a downlink physical data channel, is transmitted during a subframe section remaining after excluding the section where the downlink control channel is transmitted. A Demodulation Reference Signal (DM-RS) is a reference signal that a terminal refers to for demodulating a PDSCH.
It is designed that a signal associated with two CSI-RS antenna ports is transmitted from each of the locations 200 through 219 of FIG. 2. That is, a base station transmits, to a terminal, a signal associated with two CSI-RS antenna ports for measuring a downlink from a location 200. An antenna port is a logical concept, and a CSI-RS is defined for each antenna port and is operated to measure a channel status of each antenna port. When an identical CSI-RS is transmitted from a plurality of physical antennas, a terminal may not distinguish each physical antenna and may recognize them as a single antenna port.
In general, a CSI-RS and a cell are in a one-to-one correspondence. That is, in the case of the mobile communication system formed of a plurality of cells, as shown in FIG. 1, a CSI-RS is transmitted by allocating a separated location for each cell. For example, a CSI-RS for the first cell 100 of FIG. 1 is transmitted from the location 200 and a CSI-RS for the second cell 110 is transmitted from the location 205. Also, a CSI-RS for the third cell 120 is transmitted from the location 210. As described above, allocation of time and frequency resources for CSI-RS transmission executed in different locations with respect to each cell is to prevent interference between CSI-RSs of different cells. However, in the case of FIG. 1, transceiving antennas of each base station are disposed intensively in the center of a cell and thus it is limited in that a high data transmission rate is not supported to terminals that are disposed far from the center of the cell.
FIG. 3 is a diagram of a system to which a Coordinated Multi-point operation (CoMP) is applied, which is a multiple cell cooperative communication technology. Referring to FIG. 3, a mobile communication system includes three cells 300, 310, and 320. A transceiving antenna 330 is disposed in the center of each cell 300, 310, and 320, and distributed antennas 360, 370, 380, and 390 are disposed in different locations of a cell. The central antenna 330 disposed in the center of each cell 300, 310, and 320 may transmit a signal to a terminal with a relatively high transmission power, and forms a macro cell which has a broad coverage. The distributed antennas 360, 370, 380, and 390, which are located to be distributed in each macro cell 300, 310, and 320, may transmit a signal to a terminal with a relatively low transmission power and thus, may form a small cell having a narrow coverage. Each of the central antenna 330 and the distributed antennas 360, 370, 380, and 390 are formed of a single or a plurality of antennas.
As described above, a set formed of a single or a plurality of antennas, which is disposed in an identical point, is referred to as a point. The point is classified into a Transmission Point (TP) from the perspective of a signal transmission of a base station and a Reception Point (RP) from the perspective of a signal reception of the base station.
Referring to FIG. 3, the first cell 300, the second cell 310, and the third cell 320 correspond to a macro cell. A first-1 cell 302, a first-2 cell 304, a first-3 cell 306, and a first-4 cell 308 form a small cell in the first cell 300. A second-1 cell 312, a second-2 cell 314, and a second-3 cell 316 form a small cell in the second cell 310. A third-1 cell 322, a third-2 cell 324, a third-3 cell 326, and a third-4 cell 328 form a small cell in the third cell 320.
All of the central antenna (or a central point) 330 of the macro cell and the plurality of distributed antennas (or distributed points) 360, 370, 380, and 390, included in the macro cell, are connected together to a central controller, and controlled by the central controller. Hereinafter, a CoMP scheme in which the small cell has a cell ID identical to the macro cell is referred to as a ‘first CoMP scheme’, and a CoMP scheme in which the small cell has a cell ID different from the macro cell is referred to as a ‘second CoMP scheme.’
In FIG. 3, the first cell 300 which is the macro cell includes the central antenna 330 disposed in the center of the first cell 300, a first terminal 340, a second terminal 350, the first distributed antenna 360, the second distributed antenna 370, the third distributed antenna 380, and the fourth distributed antenna 390. The first distributed antenna 360 forms the first-1 cell 302, the second distributed antenna 370 forms the first-2 cell 304, the third distributed antenna 380 forms the first-3 cell 306, and the fourth distributed antenna 390 forms the first-4 cell 308.
The central antenna 330 provides a mobile communication service to all of the terminals located in the first cell 300. However, a distance from the first terminal 340, which is provided with a mobile communication service through the central antenna 330, to the central antenna 330 is relatively farther than a distance from the second terminal 350 to the central antenna 330. Accordingly, a data transmission speed supported to the first terminal 340 through the central antenna 330 is relatively low.
In general, as a transmission path of a transmission signal increases, a signal quality of the signal decreases. Accordingly, a mobile communication service is provided by disposing a plurality of base station distributed-antennas in a cell and selecting an optimal base station distributed-antenna based on a location of a terminal and thus, a data transmission speed may be improved. For example, the first terminal 340 communicates with the fourth distributed antenna 390 which has the best channel environment, and the second terminal 350 communicates with the first distributed antenna 360 having the best channel environment and thus, a data service of a relatively higher speed may be provided.
In this instance, the central antenna 330 takes charge in supporting a mobile communication service that requires a relatively broad coverage, a mobile communication service that requires a relatively robust quality, and a mobility of a terminal in cells.
The operation of a CSI-RS for measuring a channel status in a CoMP system that operates as shown in FIG. 3 will be described with reference to FIG. 2, as follows.
A CoMP system allocates a separate location for each macro cell or each small cell and transmits a CSI-RS so as to distinguish cells, including the macro cell and the small cell. For example, a CSI-RS for the first cell 300 of FIG. 3 is transmitted from the location 200. Also, a CSI-RS for the second cell 310 is transmitted from the location 205. Also, a CSI-RS for the third cell 320 is transmitted from the location 210. Also, in the case of the small cell included in the macro cell 300, a CSI-RS for the first-1 cell 302 is transmitted from the location 202. A CSI-RS for the first-2 cell 304 is transmitted from the location 206. A CSI-RS for the first-3 cell 306 is transmitted from the location 214. A CSI-RS for the first-4 cell 308 is transmitted from the location 216. As described above, allocation of time and frequency resources for CSI-RS transmission executed in different locations with respect to each of the macro cell and the small cells is to prevent interference between CSI-RSs of different cells.
As described above, the CSI-RS is defined for each antenna port of a logical concept. Accordingly, when an identical CSI-RS is transmitted from a plurality of physical antennas, a terminal may not distinguish each physical antenna irrespective of the geographical locations, and may recognize them as a single antenna port.
A Downlink (DL) CoMP scheme for improving a performance of a downlink, which is a wireless connection from a base station to a terminal, may include Joint Transmission (JT), Dynamic Point Selection (DPS), a Coordinated Scheduling/Beamforming (CS/CB) scheme, or a combination thereof. JT refers to a scheme in which a plurality of points simultaneously transmit signals to a terminal using an identical resource. DPS refers to a scheme in which a single Transmission Point (TP) transmits a signal to be transmitted to a terminal, and the TP dynamically varies. CS/CB refers to a scheme in which a single TP transmits a signal to be transmitted to a terminal, and a plurality of points cooperate and perform scheduling and beamforming.
An Uplink (UP) CoMP scheme for improving a performance of an uplink, which is a wireless connection from a terminal to a base station, may include Joint Reception (JR), Dynamic Point Selection (DPS), Coordinated Scheduling/Beamforming (CS/CB), or a combination thereof. JR refers to a scheme in which a signal transmitted from a terminal is received at a plurality of points, at the same time. DPS refers to a scheme in which a signal transmitted by a terminal is received at a single point, and the Reception Point (RP) dynamically varies. CS/CB refers to a scheme in which a single point receives a signal transmitted by a terminal, and a plurality of points cooperate and perform scheduling and beamforming.